Thyroid diseases represent the most common endocrine abnormalities and it has been estimated that about 20 million levothyroxine prescriptions are dispensed to about 1.8% of the population in the United States annually. Optimal use of valid laboratory tests for the assessment of thyroid status is thus important both medically and economically. In a multi-institute collaborative study, we continued to assess the performance of thyroid function tests for monitoring and the potential clinical utility of thyroid hormone therapy for suppressing papillary and/or follicular thyroid cancer. Long-term thyroid hormone therapy aiming at the suppression of serum thyrotropin (TSH) has been traditionally used in the management of well-differentiated thyroid cancer. However, formal validation of the effects of thyroid hormone suppression therapy through randomized controlled trials is lacking. Additionally, the effect of TSH at low ambient concentrations upon human thyroid tumorigenesis remains unclear. We identified 28 clinical trials from the literature that dealt with the use of thyroid hormone suppression therapy in patients with thyroid cancer between 1934 and 2001. Applying a likert scale, 15 of the17 "interpretable" studies showed either a "likely" or "questionable" beneficial effect of thyroid hormone suppression therapy on the outcome of thyroid cancer. We quantitatively evaluated the effect of thyroid hormone suppression therapy on the likelihood of major adverse clinical events (disease progression/recurrence and death) in a cumulative thyroid cancer cohort from 10 studies that qualified for meta-analysis. The 10 studies represented a combined total of 4,174 patients of whom 2,880 (69%) were reported as being on thyroid hormone suppression therapy. According to meta-analysis, the group of these patients with thyroid hormone suppression therapy had a decreased risk of major adverse clinical events (relative risk 0.73, confidence interval 0.60-0.88, p less than 0.05). Thus, despite the known adverse effects of long-term administration of thyroid hormones, suppression therapy with these hormones is justified in patients with thyroid cancer. High-sensitivity TSH assays that are capable of detecting very low TSH levels (at least less than 0.10 mU/L and, possibly, less than 0.01 mU/L) are needed to assess the adequacy of thyroid hormone suppression therapy in these patients. In another collaborative project, we analyzed the results of a pilot trial that evaluated the effects of extreme changes in thyroid hormone availability on serum lipids, lipoproteins, apolipoproteins, and on the hemostasis in thyroid cancer patients receiving thyroid hormone suppression therapy. Thyroid function tests, serum lipid parameters (with special respect to lipoprotein[a]) and various hemostatic parameters were assessed before, at, and after the time these patients were undergoing scanning. Better understanding of the relationship between thyroid hormones and the lipid and hemostatic system is important because hypothyroidism is known to be associated with elevated serum lipids and increased incidence of atherothrombotic events.